[CZ Method-Based Single Crystal Ingot Production Device]
A method for pulling a single crystal by a Czochralski method (CZ method) is a known technology, and a CZ method-based single crystal ingot production device is widely used. To obtain a single crystal by a CZ method, a single crystal is pulled out of a melt. Some single crystal ingot production devices, which can pull a single crystal faster by increasing the temperature gradient of a single crystal near the solid-liquid interface, have been proposed (Japanese Patent Application Laid-Open No. 63-256593, Japanese Patent Application Laid-Open No. 8-239291, and U.S. Pat. No. 2,562,245), and have been commercialized.
FIG. 17 is a longitudinal section depicting an example of a conventional single crystal ingot production device. As FIG. 17 shows, the conventional production device 10 comprises a heat shielding element 12 for shielding radiating heat from the liquid level of the melt 15 and the heater 16 surrounding the single crystal ingot 11, and a cooler 13 for cooling the single crystal ingot during pulling (hereafter single crystal pulling ingot). The cooler 13 is disposed to increase the temperature gradient of the single crystal pulling ingot 11 in the axis direction, and is now used for many CZ method-based single crystal ingot production devices to improve the production efficiency of a single crystal ingot by increasing the pulling speed of the ingot 11.
In such a single crystal ingot production device, a so called “hot zone” in the furnace must be disassembled and cleaned before starting the next production step when all the production steps of the ingot complete and the ingot is taken out of the furnace. For an operator to start the disassembly operation, the hot zone must be sufficiently cooled, and for this cooling it normally takes about six hours for a conventional device, which increases the time per production cycle of a single crystal ingot, and drops production efficiency.
[Oxide Precipitate]
In the silicon single crystal produced by a Czochralski method (CZ method), oxygen freed from the crucible is dissolved during crystal growth. The dissolved oxygen in the crystal supersaturates as the crystal cools down, and precipitates during the heat treatment step of the device process, generating oxide precipitates in the silicon wafer. These oxide precipitates negatively affect the leak characteristics near the surface layer, but oxide precipitates which exist in the bulk function as a gettering site, which captures heavy metal, Fe and Cu, etc., which has a negative influence on the device yield. Therefore in a silicon wafer product, it is preferable that oxide precipitates exist moderately in the bulk without existing in the surface layer portion, and functioning as a gettering site of heavy metal.
For this reason, oxide precipitates in the surface layer portion are eliminated by performing hydrogen annealing processing on silicon wafers (Japanese Patent Application Laid-Open No. 61-193456). However, this method alone is insufficient for a quality silicon wafer, and the density of oxide precipitates in the bulk and the uniformity of radial distribution are demanded, which is one of the important characteristics of a silicon wafer.
[Perfect Crystal]
Crystal defects generated during the growth of a CZ silicon single crystal obtained by a Czochralski method (CZ method) negatively affect the reliability of the gate oxide film of an MOS device and on PN junction leak characteristics. Therefore, it is necessary to decrease such crystal defects as much as possible, and conventionally a method of annealing crystals during crystal growth as much as possible has been used (e.g. Japanese Patent Application Laid-Open Nos. 10-152395, 8-12493, 8-337490). With this method, however, the decrease in defects is limited, and defects become huge.
For another approach to decreasing crystal defects, Horai et al proposed a method of eliminating defects by adjusting the relationship between the growth speed of crystals and the temperature gradient in the pulling axis direction to be a ratio in a special range, and reported that perfect crystals (defect free crystals) were obtained, which contain no grown-in defects, by this method (1993, 54th Applied Physics Society Academic Lectures (Sep. 27–30, 1993), 54th Applied Physics Society Academic Lectures, Preliminary Reports, No. 1, p. 303, 29a-HA-7; Japanese Patent Application Laid-Open No. 8-330316; Japan Crystal Growth Society Journal, Vol. 25, No. 5 (1998), p. 207).
With this method proposed by Horai et al, however, manufacturing to obtain defect free single crystals is extremely difficult depending on the growth conditions. In other words, when producing perfect crystals (defect free crystals) by a method proposed by Horai et al, the relationship between the growth speed of crystals and the temperature gradient in the pulling axis direction must be controlled to be a ratio in an extremely narrow range, which drops the production efficiency. Also, when an ingot of a silicon single crystal is produced setting the conditions to the range proposed by Horai et al, the portion of a perfect crystal (defect free crystal) is relatively small, and in terms of a stable supply of silicon wafers without grown-in defects in the industrial process, this method has the problem of uncertainty.
[Non-Uniformity of Oxide Precipitation in Perfect Crystal]
Generally speaking, a perfect crystal is a crystal where such crystal defects as voids and dislocation clusters do not exist, and a perfect crystal is also called a “defect free crystal”. In such a perfect crystal, neither grown-in defects such as voids nor the above mentioned oxide precipitates exist, but oxide precipitate nuclei do exist, so if a perfect crystal silicon wafer sliced from a perfect crystal ingot is heat treated, oxide precipitates are introduced into the wafer.
The reason why oxide precipitates are introduced into a perfect crystal wafer by heat treatment is probably because oxide precipitates are generated in the wafer by the growth of oxide precipitate nuclei along with the heat treatment of the wafer, and in a perfect crystal, the wafer radial distribution of oxide precipitation may be very non-uniform in some cases.
In other words, two zones, that is, a “vacancy dominant zone” where precipitation easily occurs, and an “interstitial silicon dominant zone” where precipitation rarely occurs, exist in a perfect crystal, and if these zones coexist in the wafer plane, the distribution of the oxide precipitation becomes non-uniform. Since the non-uniform distribution of oxide precipitation brings out a negative effect on the yield of the device, it is necessary to solve non-uniformality and to create a uniform status here by using some means.
However, it is almost impossible to adjust the growth conditions of a perfect crystal to solve such non-uniformity because the growth conditions of a perfect crystal are in an extremely narrow range, and manufacturing a perfect crystal where oxide precipitation distribution is uniform is very difficult.
[Prior Art Related to Factors of Non-Uniformity of Oxide Precipitation]
The above mentioned non-uniform distribution of oxide precipitation is probably because the density distribution of point defects, which strongly relates to the generation of oxide precipitate nuclei, is non-uniform. A typical known phenomena of difference in precipitation behavior due to point defect distribution is that vacancy is dominant and oxide precipitation tends to occur in a zone inside the OSF ring, and interstitial silicon is dominant and oxide precipitation rarely occurs in a zone outside the OSF ring.
Kissinger et al reported that the oxide precipitate density becomes uniform in the wafer plane by increasing the temperature of a silicon wafer where the OSF ring exists in the wafer surface (therefore the vacancy dominant zone and interstitial silicon dominant zone coexist) from 500° C. to 1000° C. at a 1° C./min. temperature rise speed, and then performing one hour of heat treatment at 1000° C. (Electrochemical Society Proceedings, Vol. 98-13, p. 158).
This paper, however, does not report on a perfect crystal, and in this case, void defects, due to the coagulation of voids, exist inside the OSF ring, and dislocation clusters, due to the coagulation of interstitial silicon, exist outside the OSF ring, so the method disclosed in this report cannot be directly applied to a perfect crystal. In other words, the report by Kissinger et al is a method which can be applied to a wafer where the vacancy dominant zone and the interstitial silicon dominant zone are clearly separated by the OSF ring, but cannot be directly applied to a perfect crystal where the vacancy dominant zone and the interstitial silicon dominant zone coexist in the wafer plane.
For example, if the heat treatment reported by Kissinger et al is performed on a perfect crystal, and if the oxide density is relatively high, oxide precipitation distribution becomes uniform but precipitates are generated even in the device activation layer zone in the surface layer. If the oxygen density is low, on the other hand, the oxide precipitation distribution in the plane cannot be uniform. And if oxide precipitates are generated even in the device activation layer zone in the surface layer by setting the oxygen density high, this will negatively affect the device yield, and industrial implementation is very difficult. In the case of low oxygen density where the DZ layer (non-oxide precipitation layer on the surface layer) can exist, on the other hand, oxide precipitation distribution cannot be uniform, so it is difficult to apply the heat treatment method reported by Kissinger et al for industrial implementation.
In the Japanese Patent Application Laid-Open No. 8-253392, a method for controlling density at the center of a generating oxide precipitate nuclei in the single crystal silicon is proposed, where a single crystal silicon is annealed at a minimum temperature of about 350° C., the single crystal silicon is heated (or cooled) to a first temperature 1, which is about 350° C.–500° C., during this annealing step, then this temperature is increased from T1 to a second temperature T2, which is about 500° C.–750° C., with an average speed of the temperature increase from T1 to T2, at less than 25° C. per minute, and the annealing is ended when the generation center of the oxide precipitate nuclei can be dissolved at a temperature of less than about 1150° C. According to this method, precipitates with a uniform density can be introduced using samples having different oxygen densities.
This method, however, is to make the oxide precipitate density to within about a 1 digit range (uniforming) by the heat treatment of samples with different oxygen densities, and not for solving the non-uniformity of precipitation due to the difference of point defect distribution generated in the crystal growth stage. Therefore, with this method, it is difficult to achieve a uniformity of oxide precipitation behavior in the diameter direction of crystals or in the plane of the wafer, and it is not possible to stably manufacture wafers having a DZ layer with uniform oxide precipitates. Another problem of this method is that the complicated thermal process takes time and labor, which considerably aggravates the productivity of products.